There are certain rules for determining the number of significant figures. These are stated below:

(1) All non-zero digits are significant. 

(2) Zeros preceding to first non-zero digit are not significant. Such zero indicates the position of decimal point.

(3) Zeros between two non-zero digits are significant.

(4) Zeros at the end or right of a number are significant provided they are on the right side of the decimal point. 

Hence, 2.005 has four significant figures.